This invention relates to tunable semiconductor lasers and methods of making and using the lasers. In addition, the invention relates to tunable semiconductor lasers with grating reflector structures and methods of making and using the lasers.
Semiconductor lasers have become an important component in a variety of devices and areas of use. One significant example of use is in optical communications devices and networks. One useful aspect of many semiconductor lasers is their tunability. Semiconductor lasers may be tuned using a variety of methods including, for example, the use of a grating reflector structure. For a variety of lasers, including those that are described as Distributed Bragg Reflector (DBR) lasers, the grating reflector structure and the gain region are separate elements of the laser. An example of a DBR semiconductor laser (50) is illustrated in FIG. 1, showing a gain element 52 and a tuning element 54 with a grating reflector structure 56. The grating reflector structure 56 reflects at least a portion of light having a wavelength that corresponds approximately to a multiple of the optical length of the period of the grating reflector structure and transmits light of other wavelengths. The reflected light is returned through the gain region 57. A portion of the reflected light can then be emitted as a laser beam 58. Conventionally, the laser beam 58 is emitted through a cleaved facet 59 of the semiconductor material that forms the laser. The cleaved facet 59 operates as an output coupler, also reflecting a portion of the laser light, thereby forming the laser cavity with the grating reflector structure 56.
The cleaved facet 59 is typically formed by cleaving the semiconductor material along a crystal plane and removing the surrounding semiconductor material. Use of a cleaved facet as the output coupler does not, however, permit integration of the laser 50 with other components, such as a modulator or amplifier, on the same semiconductor substrate. These other components are manufactured separately and then mounted in a desired alignment relative to the laser. The laser beam 58 propagates through free space from the laser 50 to the other components.
As an alternative, a semiconductor laser 60 can be formed in which the laser beam 70 is emitted from the tuning element 64 instead of the gain element 62, as illustrated in FIG. 2. A cleaved facet is not necessary at the end of the tuning element 64 because the grating reflection structure 66 provides sufficient reflectivity to form the laser cavity with the end reflector 68. The end reflector may be a cleaved surface or, optionally, may be a reflective coating 68 provided on the cleaved facet to enhance reflection from the end of the cavity.
The reflectivity of the grating structure 56 in the laser 50 illustrated in FIG. 1 may be relatively high, since the output from the laser is taken through the cleaved facet 59. High reflectivity on the back reflector enhances the output power. On the other hand, in the laser 60 illustrated in FIG. 2, the reflectivity of the grating reflective structure 66 is substantially lower, so as to permit efficient output of the laser beam 70.
A relatively long grating reflective structure is required for high wavelength selectivity: a narrow reflection bandwidth leads to single mode operation. There is, however, some absorption associated with a grating reflective structure, which varies when the grating reflective structure is tuned. This is not a significant problem for the laser 50 having a cleaved facet output coupler, since the variable losses are made up for by amplification in the gain element 57. This can be a significant problem, however, in the laser 60 that uses the grating reflector as the output coupler, since the power of the output beam can become unacceptably low due to absorption losses in the grating reflector. Furthermore, since the absorption losses in the grating reflector tune with the reflection wavelength of the grating reflector, the output power from the laser also becomes dependent on the operating wavelength of the laser.
Therefore, while the use of a tunable grating reflector as the output coupler permits the laser to be integrated with other components, the laser performance is compromised.
Generally, the present invention relates to tunable semiconductor lasers and methods of making and using the tunable semiconductor lasers. The devices and methods illustrated herein can provide a semiconductor laser that can be integrated with other components on a single semiconductor substrate, if desired, without requiring that the laser beam be emitted from the tuning element. In at least some embodiments, this results in good output power and wavelength selectivity. In general, the invention is directed to providing a wideband grating reflector as the output coupler of the laser.
One embodiment of the invention is directed to a tunable semiconductor laser that includes a gain region, including an active waveguide, disposed on a substrate. A tuning region is also disposed on the substrate and has a tunable, wavelength-selective reflector disposed to reflect light emitted from a first end of the active waveguide. The tuning region is tunable over a laser wavelength tuning range. An output coupler is disposed on the substrate to reflect a portion of light received from a second end of the active waveguide. The output coupler includes a wide bandwidth grating reflector structure having a reflection bandwidth approximately equal to the laser wavelength tuning range.
Another embodiment of the invention is directed to a method of operating a tunable semiconductor laser, that includes coupling light out of a tunable semiconductor laser using a wide bandwidth grating reflector structure having a reflectivity bandwidth substantially equal to a laser wavelength tuning range.
Another embodiment of the invention is directed to a tunable semiconductor laser that includes a substrate, amplifying means for amplifying light disposed on the substrate and reflecting means for selectively reflecting light at a particular wavelength back to the amplifying means. The laser also includes grating output coupling means for coupling light out from the amplifying means, wherein a reflectivity bandwidth of the grating output coupling means is approximately equal to a laser wavelength tuning means.
Another embodiment of the invention is directed to a communications system that includes an optical transmitter unit, an optical receiver unit, and a fiber optic communications link coupled between the optical transmitter unit and the optical receiver unit. The optical transmitter unit has a laser that includes a substrate, with a gain region disposed thereon, the gain region including an active waveguide. A tuning region is also disposed on the substrate and includes a tunable, wavelength-selective reflector disposed to reflect light emitted from a first end of the active waveguide. The tuning region is tunable over a laser wavelength tuning range. The laser also includes an output coupler disposed on the substrate to reflect a portion of light received from a second end of the active waveguide. The output coupler includes a wide bandwidth grating reflector structure having a reflection bandwidth approximately equal to the laser wavelength tuning range.
Another embodiment is directed to a tunable semiconductor laser, that includes a gain region disposed on a substrate, the gain region including an active waveguide. The laser also includes a tuning region disposed on the substrate, where the tuning region comprises a tunable, wavelength-selective reflector disposed to reflect light emitted from a first end of the active waveguide. A reflection spectrum of the tuning region has at least a two reflection peaks separated by a peak wavelength separation. An output coupler is disposed on the substrate to reflect a portion of light received from a second end of the active waveguide. The output coupler includes a wide bandwidth grating reflector structure with a second reflection bandwidth wider than the peak wavelength separation.
Another embodiment of the invention is directed to a method of operating a tunable semiconductor laser. The method includes coupling light out of a tunable semiconductor laser using a wide bandwidth grating reflector structure having a reflectivity bandwidth wider than a separation between reflectivity peaks of the tuning region of the laser.
Another embodiment of the invention is directed to a tunable semiconductor laser that includes a substrate, amplifying means for amplifying light disposed on the substrate, and reflecting means for selectively reflecting light at a particular wavelength back to the amplifying means. The tunable semiconductor laser also includes output coupling means for coupling light out from the amplifying means, wherein a reflectivity bandwidth of the output coupling means is wider than a separation between reflection peaks of the reflecting means.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.